1. Field of the Invention
The invention relates to a ceria-zirconia-based composite oxide, a method for producing the composite oxide, and an exhaust gas purification catalyst that includes the ceria-zirconia-based composite oxide.
2. Description of Related Art
Composite oxides containing various metal oxides have hitherto been used as supports, promoters and the like for exhaust gas purification catalysts. Ceria has been used as a metal oxide within such composite oxides because it is capable of absorbing and releasing oxygen (has an oxygen storage capacity (OSC)) according to the partial pressure of oxygen in the atmosphere. In recent years, various types of composite oxides containing ceria have been studied and a variety of ceria-zirconia-based composite oxides and processes for their production have been disclosed.
For example, Japanese Patent Application Publication No. 2011-219329 (JP 2011-219329 A) discloses a ceria-zirconia-based composite oxide that includes a composite oxide of ceria and zirconia and a method for producing the same, the composite oxide being characterized in that the molar ratio of cerium to zirconium ([cerium]:[zirconium]) is in the range of 43:57 to 48:52 and also in that the intensity ratio between the diffraction line 2θ=14.5° and the diffraction line 2θ=29° (I(14/29 value) and the intensity ratio between the diffraction line at 2θ=28.5° and the diffraction line at 2θ=29° (I(28/29) value) determined from an x-ray diffraction pattern using the CuKα line obtained by x-ray diffractometry (XRD) after heating the composite oxide in the atmosphere for 5 hours at 1,100° C. satisfy the conditions: I(14/29)≥0.015 and I(28/29)≤0.08. According to this patent publication, it is possible to provide a ceria-zirconia-based composite oxide which has a high heat resistance and which, even after long-term exposure to elevated temperatures, is capable of exhibiting an excellent oxygen storage capacity. Recently, however, as increasingly higher properties are being required of exhaust gas purification catalysts, a desire has arisen for ceria-zirconia-based composite oxides which, by possessing both a sufficiently better oxygen storage capacity and a sufficiently higher heat resistance, are able to exhibit a sufficiently better oxygen storage capacity even after long-term exposure to elevated temperatures.
In addition, International Patent Application No. 2006/030763 (WO 2006/030763) discloses both a ceria-zirconia-based composite oxide which is a composite oxide containing cerium and zirconium and which is characterized by having (1) an oxygen release onset temperature of 380° C. or less, (2) an oxygen release amount of 485 μmol/g or more, and (3) an oxygen release amount at 400° C. of 15 μmol/g or more; and also a method of producing ceria-zirconia-based composite oxides which includes mixing a cerium source with a zirconium source in a given proportion, melting the resulting starting material mixture at a temperature not lower than the melting point, then cooling the melt to form an ingot, subsequently grinding the ingot as desired to obtain a powder, then removing strain within the powder crystals by heating, and grinding the powder more finely. However, the ceria-zirconia-based composite oxide disclosed in WO 2006/030763 does not have an adequate oxygen storage material deterioration-inhibiting effect, does not always sufficiently manifest an oxygen absorption/release function after long-term exposure to elevated temperatures, and has an insufficient durability.